This invention relates to the use of acoustic pulses for atmospheric sounding or probing. It is particularly useful in measuring the height and other characteristics of thermal inversion layers [TILs] and other discontinuities in the lower atmosphere. TILs can affect radio transmission as well the transport and/or dissipation of pollutants, while other discontinuities such as windshear and clear-air turbulence near airports can affect aircraft safety.
Another technical field to which this invention may be applied is the investigation and characterization of the acoustics of buildings or the built environment.
In this specification xe2x80x9csoundxe2x80x9d will be used as a verb meaning to acoustically probe or explore, and xe2x80x9csoundingxe2x80x9d will be used as a noun to indicate the result of acoustic probing or exploration. If it is necessary to employ xe2x80x9csoundxe2x80x9d to mean an auditory sensation or the vibrations capable of causing such sensations or, indeed, to mean the generation of such vibrations (as in xe2x80x98to make a soundxe2x80x99), the word xe2x80x9csoundxe2x80x9d will be suitably qualified to make that meaning clear.
The conventional method of sounding the lower atmosphere is to use a radio sonde carried by a balloon to telemeter temperature and moisture measurements to a ground station that is able to track the balloon or its load by radar. This can provide accurate identification of TILs and the windshear occurring in so-called xe2x80x98ductsxe2x80x99 between adjacent TILs. The measurements are useful in weather prediction, plume dispersal prediction and in the characterisation of ducts for the siting of terrestrial microwave communications and other purposes. However, disposable radio sondes, along with their associated radar tracking stations, are expensive. They are also unsuited to frequent use near airports where such soundings are most needed.
It is known to measure the height of a TIL and the wind velocity above it by directing high energy single-tone acoustic pulses upwards at a high angle of elevation and analysing the time delays and Doppler shifts in a received signal after reflection and/or refraction in the atmosphere. A sounding system of this type was published in the Australian Engineer of October 1997 and was applied for the prediction of smokestack plume dispersal. In that system, a high-energy monotone acoustic pulse was directed at an angle to the vertical and the transit time and Doppler shift were detected by a sensitive receiver placed some hundreds of meters away. In order to obtain the necessary signal-to-noise [S/N] ratio at the receiver, a transmitted acoustic pulse of some hundreds of watts was directed through a large horn (antenna). A similar large horn had to be used at the receiver because of the large attenuation of the transmitted pulses in the atmosphere. The bulky transmitter and receiver elements had to be moved about to obtain measurements at various azimuth angles to discern the direction and velocity of the wind in the layer of interest. Such a system is obviously unsuitable in built-up areas because of the level of noise generated.
It is also known to sound or investigate the acoustic properties of a concert hall by feeding acoustic test signals through loudspeakers located on stage (or at other selected locations in the hall), detecting the signals received at various specific locations in the hall (usually in the audience seating) and analysing the received signals to determine the principal reflected signals and their contributions to multipath distortions and reverberation times. Expensive and sophisticated computer analysis of the composite received signal by experts is necessary because of the highly complex nature of the received signal.
The present invention involves an acoustic sounding system wherein the component tones in a transmitted chirp are mixed, differenced, correlated or otherwise compared with the component tones in an echo chirp resulting from the reflection, refraction and/or scattering of the transmitted chirp. In this way, chirp transit times (and therefore the location of reflecting or refracting discontinuities in range) can be indicated as a frequency difference between the transmitted and the received chirps at any given instant. Furthermore, phase jitter or variation in an echo tone can be detected and displayed to indicate variation in velocity of the reflecting or refracting discontinuity with respect to the transmitter and/or receiver. The transmitted acoustic chirp can be generated by feeding a loudspeaker with an electrical input signal from the sound card of a computer (for example), while the echo chirp can be detected using a microphone that generates an electrical echo signal. Though the effectiveness of both loudspeaker and microphone can be enhanced by using suitable reflector dishes, the acoustic power required in the pulse is tiny in comparison to that required for the single-tone pulse of the art.
It will be appreciated, however, that many echo chirps will be generated by a single transmitted chirp because there will normally be many atmospheric discontinuitiesxe2x80x94or TILsxe2x80x94within range. While the comparison can be done with analog systems using known mixer circuits, they may not be able to provide the discrimination required in demanding situations. It is therefore preferable to compare the input and echo signals in the Fourier domain using DSP (digital signal processing) techniques, the Fourier-transformed digital signals being subjected to complex multiplication to yield complex sums and differences from which the difference signal is normally selected. The result can be subjected to inverse Fourier transformation to generate an amplitude vs. time series in which the amplitude coordinate is the difference component (indicative of the discreteness of the TIL discontinuity) and the time coordinate is indicative of the distance of the respective TIL discontinuity from the transmitter and receiver.
In general, the chirp should have a tonal range (ie, acoustical bandwidth) suited to the object being sounded. Low level atmospheric TILs are best sounded at the lower end of the audible range; for example, 500-5000 Hz, more preferably between 800 Hz and 3 kHz and most preferably between 1 and 2.5 kHz. On the other hand, chirps used for the sounding of concert halls will generally have a wider tonal range, or successive soundings will be made using chirps having a succession of narrow tonal ranges.
The tones in a chirp can be distributed in many ways. Most commonly, the frequency of the tones will increase or decrease linearly from the start to the end of the chirp. In this case, it is desirable to attempt to achieve a uniform rate of phase-shift from the start to the end of the chirp. Such linear chirps are more easily processed, especially using analog techniques. However, many other tonal sequences can be employed. For example, the frequencies can vary in a cosine manner, in steps or even in a random or pseudo-random manner. It is practically essential to process more complex chirps of this type using DSP and Fourier techniques.
Generally speaking, the longer the duration of a chirp the greater the potential processing gain of the system when using DSP and Fourier techniques. However, the processing power required to handle Fourier transformations and Fourier domain manipulations is also positively related to chirp duration. We have found that current readily available FFT algorithms, chips and DSP techniques known in the art cannot handle chirps much longer than about 30 s duration in a practical manner. New generation FFT chips and techniques are likely to allow chirps of more than a minute to be processed.
Another consideration affecting the duration of the chirp is whether the echo signals are to be processed in real-time or off-line. The simplest approach is to process the echo signals in real-time and to make the transmitted signal (and chirp) of sufficient duration to ensure that echo signals start arriving before the input signal has finished. In this way, the frequency difference between the tones being transmitted and received (from reflection) at any instant is indicative of the distance of the TIL causing the reflection (for a linear chirp), and, the duration of the chirp will determine the range within which TILs (or other targets) can be detected.
Comparison of the input and echo signals off-linexe2x80x94ie, not in real timexe2x80x94offers the advantage that the range of distances from which echoes are generated can be selected. Either or both the input and the echo signals can be recorded (before or after digitization and transformation) and then jointly played back with the desired time-offset to effect their comparison. For example, if signal processing considerations limit the chirp length to 15 s so that the maximum height at which TILs can be reliably detected in real time is, say, around 5000xe2x80x2, the input signal can be delayed by, say, 15 s after the transmission of the acoustic chirp, so that TILs in the range of 5000 to 10000xe2x80x2 can be detected using real-time echo signals by comparing the delayed signal with the echo signals arriving between 15 and 30 s after the start of the acoustic chirp.
While it will be normally desirable for the transmitted acoustic energy to be uniform over the chirp duration, or the same for each tonal increment of the chirp, the energy may be varied with respect to tone in order to compensate for anticipated frequency-selective attenuation in the environment being probed.
As indicated above, a convenient method of generating the chirp is to feed appropriate software (eg, MIDI commands) to a PC sound card so that the desired tone sequence can be generated upon command if a linear chirp is to be used, this technique allows the tone increments to be sufficiently small to create the effect of a continuous phase-shiftxe2x80x94or smooth glissandoxe2x80x94from beginning to end of the chirp. This input signal can be stored in a sound (wave) file in the PC and used generate repeated chirps at any desired time interval and, as already indicated, this input signal can be transmitted to a mixer for comparison with the echo signal at any desired time. Of course, acoustic chirps should not be transmitted so frequently that echoes from multiple chirps are received at the same time. If desired, a Fourier transformed input signal may be stored in the PC so that it can be fed to the comparator at the appropriate time for mixing with the transformed echo signal. This technique can reduce the real-time processing burden.
It will be appreciated that there will necessarily be direct transmission of the signal pulse from transmitter to receiver via the shortest route, as well as some indirect reflections from terrestrial objects. These xe2x80x98directxe2x80x99 pulses may overlap the desired echoes in time at the receiver and degrade echo detection and processing. The direct pulse can be attenuated by acoustically isolating the transmitter and receiver, but this is often difficult or inconvenient. It can be subtracted from the echo chirp using known DSP techniques but, if the overlap of the direct and echo chirps is not great for the echoes of most interest, processing in the Fourier or frequency domain can effectively remove or discount most direct chirps. If the direct signals are not removed, the resulting amplitude-time display will show early high-amplitude returns that can be readily ignored in most cases.
The techniques of this invention will be of great assistance in identifying TILs that act as graded-index refractors and ducts that bend, reflect or channel microwave signals. Since these TILs and ducts tend to form within a few hundred meters of the ground in a generally predicable pattern for a given location and season. As such TILs tend to be ripple, their characterization can be vital for the optimal location and design of microwave links. A rippling reflective TIL (one that has short-term localized vertical velocities) that forms above the path of a telecommunications microwave beam will generate rapidly fluctuating multi-path signals at the microwave receiver, causing signal fading and data loss. Using the techniques of the invention, the rippling of the TIL can be displayed as phase jitter. Where short-term vertical air movements are important (eg, when atomospheric turbulence is of concern), the phase jitter may be the main subject of the sounding.
Thus, use of the techniques and apparatus of this invention for low-level atmospheric soundings permits the identification and quantification of windshear and CAT (clear air turbulence). In this context, windshear indicates the relatively sudden change in direction or velocity of wind with a relatively small change in elevation. Such changes most usually occur at TILs and it is quite common for a layer of wind that is sandwiched between upper and lower TILs, to have propertiesxe2x80x94such as velocity, speed, direction, temperature, moisture content or the likexe2x80x94that differ markedly from those of the air bodies above and below the TILs. Windshear can be a problem for aircraft if it is severe and in the vicinity of a runway. While CAT can be regarded as a special case of windshear, the term is generally reserved for localised non-layered turbulence. Such turbulence has been reported in the wake of large jet airliners as they approach landing or after they take off. CAT of this nature has been blamed for the crashes of light planes landing or taking off immediately following a large airliner. Since CAT is evidenced by local variations in air density and/or temperature as well as velocity, it will refract and reflect beamed acoustic pulses and, therefore be amenable to identification and quantification using the methods and apparatus of the present invention.
The use of chirped acoustic sounders aligned with and cross-runway such that the chirped pulses are directed at a low elevation allows windshear and CAT in the vicinity of a runway to be identified. Preferably, mirrored transmitter and receiver sets are used in each direction so that pulses can be transmitted first in one direction and then in the other. The height of the windshear or CAT can be estimated by the time delay between transmission and reception of the reflected or refracted pulses, while the velocity of the associated body of air in the pulse-beam direction can be estimated by comparing the differential time shifting of the xe2x80x98upxe2x80x99 and xe2x80x98downxe2x80x99 pulses. The component time-delay measurements that allow the differential comparison can each be obtained using the techniques indicated above, yielding a highly accurate measurement of wind velocity or turbulence at any desired height within range. In general, chirp transit-time measurements conducted in this way will be more accurate than Doppler-based measurements.
As also mentioned above, the chirped acoustic pulse techniques offer substantial benefits in the characterisation of concert hall acoustics, though the chirp duration will generally be shorter and the chirp bandwidth higher than for atmospheric sounding. This allows highly precise measurement of the length of the principal multiple paths between a chirped pulse transmitter at a given location to a given receiver location, which then allows computation of multi-path interference for those locations over the range of audible tones. Such measurements and computations then provide most valuable inputs into the correction or optimisation of an existing concert hall.
The invention can be embodied in apparatus, systems or methods for acoustic sounding in air.